Identification system

ABSTRACT

An identification system, consisting of at least one stationary element and at least one movable element or token, the first element having a first induction coil adapted to generate a magnetic alternating field and being connected to a source of alternating current, and to a detector for detecting variations of the electromagnetic field, the detector being adapted to compare the detected patterns, the second element having a second induction coil adapted to pick up the field of the first coil and being connected to a rectifier circuit adapted to rectify the induction voltage of the second coil, a capacitive storage device being provided for smoothing the rectified voltage, a control circuit being fed by the rectified voltage and receiving, at an input terminal, the ac voltage induced in the second coil, the control circuit having an encoding deive adapted to produce, at an output terminal, code signals identifying the second element, these code signals being used for actuating a switch which, in one condition, short-circuits the second coil. This system is characterized in that the capacitive storage device is of a small capacity, in particular an integrated or intrinsic capacity of the circuit of each second element, in that the rectifier is a full-wave pg,2 rectifier bridge, two opposite corners thereof being connected to the respective ends of the second coil and one thereof also being connected to the signal input terminal of the control circuit, and in that the switch means is adapted to short-circuit one arm of the bridge which is connected to that end of the second coil which is not connected to the signal input terminal of the control circuit.

BACKGROUND

The present invention relates generally to the field of electroniclocking or access-control systems where a user wishing to gain access toa restricted area, or to some service such as the use of acash-dispensing machine, presents to a receiving apparatus a token whichtransmits identification data indicative of the user's authority to gainsuch access. More particularly, the invention relates to acontactless-identification system of the kind where the receivingapparatus includes an alternating magnetic field generator and the tokenincludes a transponder powered by induction from said field.

SUMMARY

As essential component of the transponder in a system of the kindindicated above is a coil or antenna through which the power foroperation is induced and through which the identification data istransmitted. In prior art locking system in which this principle ofoperation has been used at least the antenna (and possibly othercomponents, in particular capacitors) is manufactured and assembled as adiscrete component separate from the rest of the transponder circuit, towhich it must be electrically connected when assembled to the token. Thepresent invention, on the other hand, proposes in one broad aspect atoken for use in an electronic locking or access-control system of thekind indicated above of which the transponder is manufactured as acomplete integrated circuit including the antenna. By eliminating theneed to make connections between at least a separate antenna and otherparts of the transponder circuit the reliability and manufacturing costof a token in accordance with the invention should be respectivelyincreased and decreased in comparison with those used in prior artsystems.

BRIEF DESCRIPTION OF DRAWINGS

This and other features of the present invention will now be moreparticularly described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic block diagram of a system operating in accordancewith the invention as applied to physical access-control;

FIGS. 2-6 are schematic block diagrams of various circuits illustratingthe development of a suitable transponder in accordance with theinvention;

FIG. 7 illustrates schematically an example of the implementation oftransponder as an integrated circuit; and

FIGS. 8A-8D include several schematic block diagrams illustrating thedevelopment of a suitable transmitter for use in the system.

DETAILED DESCRIPTION OF DRAWINGS

Referring to FIG. 1, the illustrated access-control system comprises areceiving apparatus 1 to which users present an individualidentification token 2 when entry through a normally-locked door 3 isrequired. When a token is presented to the apparatus its presence issensed by a detector 4 which instructs the central logic unit 5 toactivate a transmitter 6. The latter energises the primary coil 7 of analternating magnetic field generator comprising a ferromagnetic core 8which forms a magnetic loop with a small gap 8A. When the token 2 ispresented to the apparatus an integrated circuit transponder 9 carriedby it is placed within the gap 8A and is subjected to the alternatingfield concentrated by the core 8. As more particularly described belowthe transponder is thus caused to transmit identification data whichmodulates the magnetic field in the core 8 in amplitude or frequency, oradds a magnetic field with a different frequency, and can be detected bya receiver 10 associated with the transmitter 6. The received data iscompared by the logic unit 5 with data stored in an associated memory 11and if found valid an electromagnetic door release 12 is actuated topermit entry.

The transponder circuit 9 may operate on a principle as illustrated inFIG. 2. An antenna coil L receives the magnetic energy from thegenerator in the apparatus 1 and provides the circuit with power tofunction. Diodes D1-D4 rectify the induced voltage in L, capacitor CSbuffers the supply voltage VDD, while zener diode ZD limits the maximumsupply voltage. The frequency of the alternating magnetic field,suitably divided, acts as a clock for logic circuitry 13 which drives ashift register 14 containing the identification data from a memory 15.This information is transmitted by intermittent closing and opening of asolid state switch S. When S is closed the resonance frequency of theinput circuitry containing capacitors C1, C2 changes. This leads to avariation of the amplitude of the magnetic field and can be detected bythe receiver 10.

Problems arise, however, in integrating a circuit of this kind withcapacitors C1, C2 and CS onto a single monolithic device. In generalonly very small capacitors, with a relatively poor tolerance on thecapacitance value, can be practically integrated onto a chip because ofthe large surface area they require, and in the present instance thecoil L also will occupy a considerable proportion of the chip area. Thismeans that the supply decoupling will be very limited because of thesmall possible values of CS while the resonance frequency of the inputwill have a wide tolerance in the absence of any economical form of "onchip" trimming.

To avoid the use of C1 and C2, a non-resonating input as shown in FIG. 3could be used. Switch S short-circuits coil L, which leads to anamplitude modulation of the magnetic field, which can be detected by thereceiver 10. In addition, if the transmitter 6 consists of an oscillatorof which the frequency is (partly) determined by the inductance of coil7, modulating the magnetic field amplitude will also modulate thisinductance and therefore the frequency of the magnetic field. Detectionof this data within receiver 10 can therefore also by realised by usingFM-detection instead of AM-detection.

Two drawbacks occur, however, in that when S is closed the circuit willno longer be provided with power and also the clock signal willdisappear. Even when a series resistance R is used, the supply voltagewill still decrease significantly; otherwise, by increasing R, asignificant decrease of the modulation of the magnetic field will occur.A solution to this problem is shown in FIG. 4. Switch S nowshort-circuits the coil only during one half of a period of the magneticfield. In this way, no significant decrease of the supply voltage occurswhen S is closed. By connecting the clock input of the logic circuitwith point A, the clock signal will constantly be available whether S isclosed or not.

An alternative scheme for data-transmission from the responder isillustrated in FIG. 5 where the coil L is short-circuited during thefirst half period of the magnetic field or during the second half periodof the magnetic field, dependant on the logic state of the output. Inthis way a magnetic field with the same frequency as the output signalis added to the existing magnetic field instead of modulating thisfield, so that the AM- (or FM-) demodulator situated in the receiver 10can be replaced by a high-cut filter.

The power supply capacitor CS can be kept small by using a high supplyfrequency--say 10 MHz--and by keeping the power consumption low. Thestatic power consumption can be very low if the circuit is realised in aCMOS technology, but at high frequencies (like the used input clockfrequency) the power consumption of CMOS becomes considerable. Thisconsiderable power consumption is due to the transitions of the logiclevels inside the circuit, so if it is arranged that transitions onlyoccur on the positive going edge of the input clock, the power duringthe transitions can directly be derived from L instead of the supplycapacitor CS. During the transitions the supply current will first beobtained from CS, but because CS is small, the supply voltage VDD willdecrease rapidly, until VDD is equal to the momentary voltage on A minusthe forward voltage drop of D4. From that moment L will provide thecircuit with supply voltage and current during a large part of thepositive period of the voltage on A. A voltage divider R1, R2 might beused (as indicated in FIG. 4) to provide a higher voltage on A at themoment the clock input is 'triggered' and multiple transitions insidethe circuit occur.

A schmitt-trigger clock input will also enhance reliable operation.

A further improvement which makes detection of the transmitted dataeasier is the use of a modulator such as indicated at 16 in FIG. 6. Toexplain, when token 2 is detected by detector 4, fast retrieval of theidentification data is desirable for reasons of fast door release andminimising power consumption. When the transmitter 6 is switched on alot of low frequency components with a large amplitute occur for a whilein the received signal. This makes easy and reliable detectionimpossible for some time directly after switching on the transmitter. Tominimise this time delay a low-cut filter should be used with a cut-offfrequency as high as possible, but not higher than the lowest frequencycomponents of the signal transmitted by the responder. Therefore thelowest frequency components of the transmitted signal should be as highas possible. If the data output of shift register 14 is applied directlyto S, as indicated in FIGS. 4 and 5, it will happen that S is closed oropened for a long period if the data contains a succession of one's (orzero's next to each other. Therefore the transmitted data will containlow frequency components, which restrict the cut-off frequency of thelow-cut filter inside the receiver 10. If instead the data is modulatedonto a subcarrier (using a phase or frequency modulation principle), andthis modulated subcarrier is used to switch S, the difference betweenthe maximum and minimum time that S is closed or opened can be highlyreduced. Thus, by using the modulator 16 the lowest frequency componentscontained by the transmitted signal can become more than an order ofmagnitude higher; therefore the cut-off frequency of the low-cut filterinside the receiver 10 can be chosen an order of magnitude higher whichleads to a much faster and reliable detection of the transmitted data.

For reasons of power consumption of the transponder, VDD must be as lowas possible. This makes it desirable that the input clock divider andtherefore the whole final circuit is realised in a high speed CMOStechnology. If for other reasons, standard CMOS would be highlypreferred, an alternative circuit is possible with an on chiposcillator. Both alternatives are indicated at 17 and 18 respectively inFIG. 6.

Turning to FIG. 7, this indicates one possible implementation of theentire transponder circuit 9 on a chip which may measure approximately4×4 mm. The active components of the circuit are provided in the centralarea 19 surrounded by the turns of the antenna coil L. Contact pads foruse in programming the identification data into the memory 15 areindicated at 20. If, however, programming can be achieved by acontactless method, making use of the coil L to provide the circuit withsufficient power and to transfer the data to be programmed, area 20 canbe used for the corresponding programming logic.

The transponder circuit 9 is schematically illustrated in FIG. 1 asmounted in the shank of a token 2 shaped to resemble a conventional keyand this represents a convenient implementation of a personalidentification device. In principle, however, the structure of thesubstrate by which the transponder is carried is open to considerablevariation--and could for example be in the form of a card--so long asthe token and receiving apparatus are appropriately mutually configuredto place the transponder correctly in relation to the field generatorwhen used.

A transmitter 6 configuration which would normally be applied is shownin FIG. 8A. The magnetic fieldstrength inside the airgap 8A is to afirst approximation a linear function of the current Ip multiplied bythe number of primary windings Np. Because the system is batteryoperated, the total supply voltage and current are limited, while thepower dissipation must be kept to a minimum. This has led to thefollowing considerations:

(1) The number of windings of the coil L on the transponder chip isrestricted and must be kept to a minimum for reasons of minimising chiparea and series resistance of the coil. On the other hand the totalinduced (secondary) voltage in the coil on the chip must be maximised.Because the secondary voltage is inversely proportional to the primarynumber of windings Np, Np should be minimised. The smallest practicalvalue of Np is one (FIG. 8B).

(2) When Np is chosen to be one, the inductance and therefore theimpedance of the primary coil is very low and Ip and the power to bedelivered by the battery becomes excessive, which is not allowable. Asolution is found by using a high frequency (say 10 MHz) and by tuningthe inductance of the primary coil using a capacitor Cp (FIG. 8B).Although Ip does not change, the current Ig to be delivered by thegenerator and therefore also the current and energy to be delivered bythe battery is decreased by an order of magnitude.

(3) To obtain the stated severe decrease of the generator current Cpmust be tuned, which is undesirable during mass production. A solutionis to make the tuned network LpCp the frequency determining part of agenerator, formed by LpCp and a transconductance amplifier (FIG. 8C). Inthis way the frequency is automatically tuned to LpCp.

(4) Because the excessive current Ip still flows through the capacitorCp, this capacitor must be a high quality, low loss and therefore anexpensive and large device. A solution to this problem is shown in FIG.8D, where the primary coil is divided into two parts. The inductanceseen between point A and B is highly increased which leads to a muchlower value for Cp and Ip. Therefore Cp can become a standard typecapacitor.

I claim:
 1. An identification system, consisting of at least one firstor stationary element and at least one movable or second element ortoken, said first element comprising a first induction coil adapted togenerate a locally restricted magnetic alternating field, said coilbeing connected to a source of alternating and in particular hf current,and also being connected to means for detecting variations of saidelectromagnetic field, said detecting means being connected to signalprocessing means adapted to compare the detected field variations with aplurality of standard signal patterns, and to produce an output signalin the case of correspondence with one of said patterns, said secondelement comprising a second induction coil adapted to pick up the fieldof said first coil when placed in the field of said first coil, saidcoil being connected to a rectifier circuit adapted to rectify theinduction voltage of said second coil, a capacitive storage means beingprovided for smoothing the rectified voltage, further comprising acontrol circuit fed by the rectified voltage from said rectifier circuitand receiving, at an input terminal, the ac voltage induced in saidsecond coil, said control circuit comprising encoding means adapted toproduce, at an output terminal, code signals identifying said secondelement, said code signals being used for actuating a switch meanswhich, in one condition, short-circuits said second coil, characterisedin that said capacitive means is a small capacity, in particular anintegrated or intrinsic capacity of the circuit means of said or eachsecond element, in that said rectifier means is a full-wave rectifierbridge with four arms, two opposite corners thereof being connected tothe respective ends of said second coil, one thereof also beingconnected to the signal input terminal of said control circuit, and theother two corner points forming the supply and ground terminalsrespectively of said rectifier bridge, and in that said switch means isadapted to short-circuit one arm of said bridge which is connected tothat end of said second coil which is not connected to the signal inputterminal of said control circuit.
 2. The system of claim 1,characterised in that, in said or each second element a second switchmeans actuated by said control circuit is provided which is adapted toshort-circuit the opposite arm of said rectifier bridge, said secondswitch means being controlled by said output code signals in phaseopposition in respect of said first switch, the corner point of saidrectifier bridge connected to said first switch means being connected toa second input terminal of said control circuit.
 3. The system of claim1, characterised in that the or each input terminal of said controlcircuit is connected to a Schmitt trigger for providing shaped pulsesderived from the induced voltage to be used as clock pulses forcontrolling the encoding means.
 4. The system of any one of claims 1,characterised in that said control circuit is provided with a frequencydivider, adapted to produce clock pulses having a lower frequency thanthe hf voltage induced in said second coil.
 5. The system of any one ofclaims 1, characterised in that the code signal output of said controlcircuit is connected to a modulator which is connected to the controlterminal of said switch means.
 6. The system of any one of claims 1,characterised in that said control circuit comprises an oscillator forgenerating a clock frequency signal and in that said or each firstelement comprises synchronising means for synchronising the signalprocessing means with clock pulses derived from the field variationsdetected by said first coil.
 7. The system of any one of claims 1,characterised in that said second coil is an integrated part ofintegrated circuits of said second element or token.
 8. The system ofany one of claims 1, characterised in that said first coil is afrequency determining part of an oscillator forming the source ofalternating current, and in that said coil consists of a small number ofturns, and in particular of one single turn.
 9. The system of claim 8,characterised in that said coil having a small number of turns isinductively coupled with an auxiliary coil having a larger number ofturns than said first coil, and being connected to a capacitor formingthe other frequency determining element of said oscillator.